Milk-based powders have traditionally been produced by preparing a liquid concentrate of all the ingredients making up the powder. The liquid concentrate is then transported to a spray drier and dried to powder. This process is often known as a wet-mix process. Powders produced by this process are usually homogeneous and dissolve rapidly. However, if the liquid concentrate contains carbohydrates, the carbohydrates tend to cake in the spray-drier, causing fouling. The caked carbohydrates are extremely difficult to remove. This may cause significant cleaning downtime. For example, it may in certain circumstances by necessary to clean the spray dryer every 24 to 48 hours. A further problem may occur with certain carbohydrates such as starch which swell when in contact with water and produce a very viscous liquid concentrate which cannot easily be sprayed into the dryer. Also, if it is desired to include heat sensitive ingredients in the final product and such ingredients are added to the liquid concentrate, they will begin to degrade during the spray drying process so that it is necessary to overdose these constituents in order to be sure that a sufficient amount will remain in the final product. This is clearly undesirable from an economic point of view. One example of a heat sensitive ingredient is Vitamin C.
To overcome these problems, it is now common to produce milk-based powders in two stages. In the first stage, a base powder is produced from a liquid concentrate by spray drying. The base powder will contain most of the ingredients of the final product. However, the base powder has reduced levels of carbohydrates and heat sensitive ingredients. After the base powder has been produced, the remaining carbohydrates and any heat sensitive ingredients are added by dry mixing. The dry-mix process reduces the problems of fouling in the dryer and solves the problem of degradation of heat sensitive ingredients. However, powders produced by the dry-mix process are much less homogeneous. In particular, after storage and transport the lack of homogeneity may be visible to the naked eye. In addition to this aesthetic disadvantage, a technical consequence of the lack of homogeneity is that powders produced by dry mixing do not dissolve as rapidly or as completely as those produced by wet mixing of all constituents.
There is therefore a need for a process for producing carbohydrate-containing nutritional products by spray-drying which results in products with a good structure but with reduced levels of fouling in the spray-drier.